The control of electrical conductivity in p type AlGaN is a very difficult problem from technical and scientific viewpoints. Magnesium (Mg) is a main p type dopant for GaN and AlGaN. However, the acceptor level of Mg is about 230 mV (experimental value) in GaN and is even higher and deeper in AlGaN. As a result, the hole density of AlGaN doped with Mg becomes extremely low as compared to that of GaN also doped with Mg.
AlGaN doped with Mg (Mg-doped AlGaN) with a high aluminum composition rate is not suited to light emitting devices such as light emitting diodes (LEDs) or laser diodes because the electrical conductivity of Mg-doped AlGaN is very low. Therefore, light emitting devices such as LEDs or laser diodes using Mg-doped AlGaN with a high aluminum composition rate are very difficult to realize. Similar problems exist in the fabrication of ultrahigh high frequency or power control devices using a GaN or AlGaN nitride semiconductor.
For example, the acceptor level of magnesium (Mg) deepens as the aluminum composition rate in Mg-doped AlGaN increases. Consequently, the electrical activity of Mg drops to 1% or less as the aluminum composition rate increases, and then, the hole density of Mg-doped AlGaN becomes extremely low and the electrical resistivity thereof becomes high. In order to increase the hole density and decrease the electrical resistivity of AlGaN with a high Al composition rate, a large amount of Mg is added. However, when the concentration of added Mg is about 2×1020 cm−3 or higher, Mg causes segregation in AlGaN and then the crystalline quality drops significantly. Therefore, 2×1020 cm−3 or more of Mg cannot be doped into AlGaN. As a result, realization of ultra-violet LEDs, laser diodes, and electronic power control devices using Mg-doped AlGaN with a high Al composition rate becomes difficult.
In addition, when an AlGaN semiconductor layer is doped with Mg by the current Mg doping technology, the resultant AlGaN semiconductor layer has a low hole density and therefore has high resistivity. In the current LED structure, the layer thickness of a p type AlGaN layer can merely be increased to 0.1 μm to 0.2 μm at the maximum. In practice, it is difficult to increase the thickness of a p type AlGaN layer to 0.2 μm. In the meantime, an ultraviolet or deep-ultraviolet semiconductor laser using AlGaN with a high Al composition rate has not been realized. Consequently, the oscillating wavelength of the semiconductor laser is limited to a wavelength on the long wavelength side near the bandgap of GaN.
In addition, Mg is thermally diffused significantly. Even an attempt to form an n type layer on an Mg-doped p type layer fails because Mg thermally diffuses along defects. Therefore, npn or pnp bipolar transistors using an Mg dopant cannot be manufactured. This presents a serious obstacle against realization of electric power control devices for electric vehicles or hybrid vehicles.
As described above, there are various problems related to an Mg-doped p type AlGaN layer. For solving these problems, patent document 1 (Japanese Laid Open Patent Publication 2011-23541) discloses the following technology. A support body is used which is formed of a III-group nitride semiconductor and has a main surface having an angle of 40° or more and 140° or less with respect to a reference plane which perpendicularly intersects a reference axis extending in a c axis direction. On the main surface of the support body, a p type gallium nitride semiconductor layer containing carbon at a concentration of 2×1016 cm−3 or higher in addition to Mg is formed.
Carbon is an amphoteric dopant, and thus becomes either an acceptor or a donor depending on the material into which carbon is introduced. In the method of manufacturing a p type gallium nitride semiconductor layer doped with Mg and carbon disclosed in patent document 1, the gallium nitride semiconductor layer may occasionally become of an n type, and it is not possible to form a p type gallium nitride semiconductor layer sufficiently stably. More specifically, because the main surface of the support body formed of a III-V group nitride semiconductor has an angle of 40° or more and 140° or less with respect to the reference plane which perpendicularly intersects the reference axis extending in the c axis direction, the carbon does not function as a p type dopant stably.
The present invention made in light of the above-described situation has an object of providing a method of manufacturing a semiconductor device including a p type gallium nitride semiconductor layer doped with carbon which has a high level of reproducibility and an improved productivity. The present invention has another object of providing a p type gallium nitride semiconductor layer doped with carbon which has high electrical conductivity and low resistivity, and a semiconductor light emitting device including such a p type gallium nitride semiconductor layer. (In this specification, a layer doped with carbon or AlGaN doped with carbon will also be expressed as “carbon-doped layer” or “carbon-doped AlGaN”.)